This invention relates to a high speed data modem for transmitting and receiving data over common, band-limited telephone channels and the like, and more particularly to a modem which achieves high speed by reducing intersymbol gaps and delays without the normally attendant distortion.
Modems (an acronym for modulation/demodulation) were developed to enable transmitting digital data over low-grade, but pervasive, analog telephone voice channels. With the ability to transmit data over telephone channels, not only was the need for specially designed data transmission networks eliminated, or at least reduced, but also the number of possible originating and destination stations was greatly multiplied, limited only by the availability (or more precisely nonavailability) of a telephone line.
Modems operate by transforming binary (two-level) digital data signals into analog signals suitable for transmission over telephone channels and then, after transmission, transforming the analog signals back into the digital data signals. This conversion involves modulating or encoding the digital data onto a carrier signal or signals at the transmitting end, and demodulating or decoding the transmitted signals at the receiving end to recover the desired digital data.
Since the introduction of modems, development efforts have been directed to improving their speed and accuracy, resulting in systems capable of transmitting 9600 bits per second (see, for example, U.S. Pat. Nos. 4,206,320 and 4,771,417) and, most recently, 14,400 bits per second (see U.S. Pat. No. 4,646,305). Also see U.S. Pat. Nos. 3,955,141, 4,085,449, 4,355,397, 4,514,825, 4,653,044, 4,686,690, 4,734,920 and 4,796,279.
One of the problems which inhibits improvement in speed and accuracy of data transmission over telephone channels, at least for those systems which utilize multiple carriers (frequencies), is what is denoted "group delay distortion". This type of distortion comes about because of the difference in phase delay for different frequencies. The result of such distortion is that different frequencies of a transmitted composite analog signal arrive at the receiving end at different times, some frequencies lagging behind others, so that one signal symbol or frame may interfere with an immediately preceding or succeeding symbol, e.g., the late arriving frequencies of a symbol interfering with the early arising frequencies of a succeeding symbol.
Another problem of systems which utilize multiple carriers arises from the fact that the symbol waveforms (transmitted composite analog signal) are periodic and thus, if transmitted continuously, one immediately after the other, give rise most times to sharp discontinuities between symbols. These discontinuities, in turn, produce severe distortion (from the harmonics of the discontinuity) in the succeeding symbol.
One approach to overcoming both the group delay distortion problem and intersymbol discontinuity problem, described in U.S. Pat. No. 4,206,320 for example, is to provide a gap or guard time between symbols to thus reduce intersymbol and discontinuity distortion. That is, the demodulator at the receiving end is arranged to ignore the received signal for a portion of the baud time. Of course, this also increases the transmission time (i.e., increases delay) which, stated in other words, decreases the transmission rate.